The subject matter disclosed herein relates to power converters, such as inverters for motor control. More specifically, the inverter executes a pulse width modulation (PWM) routine and coordinates the switching periods of the PWM routine with a synchronizing signal.
As is known to those skilled in the art, a motor drive receives an input voltage and converts the input voltage to a suitable output voltage for controlling operation of a motor. In an Alternating Current (AC) motor drive, a three phase AC voltage is typically available at, for example, 230 V or 460 V as the input voltage. The motor drive includes a converter section that rectifies the AC input voltage into a Direct Current (DC) voltage. The DC voltage is present across a positive and a negative terminal of a DC bus in the motor drive. An inverter section includes switches, such as transistors, thyristors, or silicon-controlled rectifiers to convert the DC voltage on the DC bus into an AC voltage output at the desired magnitude and frequency to control operation of the motor.
The motor drive often utilizes a pulse-width modulation (PWM) routine to control the switches in the inverter section. The switches alternately connect and disconnect either the positive or the negative terminal of the DC bus to the AC output. The resulting output is, therefore, either zero volts or fully on at the voltage level present on the DC bus. In order to vary the magnitude of the output voltage, the PWM routine repeatedly executes at a predetermined interval, sometimes referred to as a carrier period, where the inverse of the carrier period is the carrier frequency. The PWM routine receives a reference signal corresponding to the desired output voltage magnitude and controls the switches such that the DC bus is connected to the output for a portion of the carrier period. Thus, during each carrier period, the output is on for a percentage of the carrier period and off for the remaining percentage of the carrier period and an average voltage magnitude for each carrier period results. By varying the percentage of the carrier period that each switch is on or off, the average voltage magnitude varies such that it corresponds to the reference signal input to the PWM routine. If the fundamental frequency of the desired AC voltage is much less than the carrier frequency, the resulting output voltage waveform approximates the desired AC voltage.
However, the high frequency switching generates undesirable AC electrical content at the carrier frequency and harmonics, or multiples, thereof. These high frequency electrical components may result in radiated and/or conducted emissions that are coupled back to the AC input voltage. In addition, motors may not always be located near the drives by which they are controlled. Using long motor leads to connect the motors and drives may result in reflected waves being established on the motor leads. Because of the high frequency of these emissions, leakage currents may be established through capacitive coupling between leads and to ground. If left unmitigated, these conducted emissions could interfere with other electrical devices receiving the same input voltage or connected elsewhere within the facility.
In order to prevent these conducted emissions from being transmitted back to the AC input voltage, a line filter is typically connected at the input of the motor drive. However, line filters add cost and require additional space near the motor drive. Further, many systems require multiple motors and multiple motor drives to control those motors, increasing the total magnitude of the conducted emissions. Consequently, either multiple filters, connected to each drive, or a single filter, having increased capacity may be required. Either option increases the cost and space dedicated to filters. Thus, it would be desirable to provide a system to reduce the total emissions generated by inverters such that smaller filters may be utilized